1. Field of the Invention
The invention concerns coaxial cable, and more particularly, a method for manufacturing a coaxial cable.
2. Description of the Prior Art
A method for manufacturing a coaxial cable is known from German document No. DE-OS 16 40 194, wherein an aluminum band or strip is formed into a large diameter tube around a foam-insulated (dielectric material-insulated) conductor. The tube is welded with a lengthwise seam and is pulled down over the layer of dielectric material. A welding torch is used to weld the lengthwise edges of the tube. An advantage of a cable manufactured in this manner is that the fully closed outer tube is a conductor which prevents radiation losses. A disadvantage is that water can propagate in a gap between the dielectric material and the outer conductor, thereby diminishing the transmission properties of the cable. Such a gap cannot be avoided, although the dielectric material is slightly compressed when the outer conductor is pulled down over it.
A coaxial cable is known from U.S. Pat. No. 3,315,025, wherein an outer conductor comprises an aluminum or copper strip wrapped longitudinally, with overlapping edges, around a dielectric layer comprising foam material which surrounds an inner conductor. The aluminum or copper strip is coated with a copolymer coating. A relatively thick outer jacket of plastic material is extruded over the outer conductor and copolymer coating, with a wall thickness of between 1 and 3 mm. The copolymer coating is activated at the extrusion temperature of the outer jacket, and provides a good adhesive bonding between the outer conductor and the outer jacket. The overlapping edges of the strip are also adhesively bonded by the copolymer coating. This cable has a disadvantage in that the outer conductor does not form a self-closing sheath, therefore energy can escape through a slot between the overlapping edges of the band. This problem is aggravated by the fact that the overlapped seam may tear apart at increased temperatures. Since plastics have a higher coefficient of expansion than metals by a factor of 10, it is possible that the overlapping seam will rupture during a long-term exposure to a temperature of 50.degree. C., for example. Another disadvantage is that the overlapped seam is copied on the jacket surface. Additionally, a lengthwise water migration along the cable cannot be avoided.